Date   : Wed, 23 Oct 2002 21:47:42 GMT
From   : pete@... (Pete Turnbull)
Subject: Re: BBC keyboard playing up

On Oct 24,  9:57, Michael Foot wrote:
> Quoting Pete Turnbull <pete@...>:
>
> The motherboard had some slight damage around the 74LS163 (IC76 below the
> keyboard pins) where a flying lead had been soldered, but I think this IC
is
> only used for the ROM paging and appears to be working ok as the ROMs are
> fine.

Yes, it's just used as a 4-bit latch.

> When the keys fail, the speaker emits very faint buzz noise
> (interference) whenever one of the bad keys is pressed. It's almost like
the
> signal is getting so far but not making it to it's destination.

That might be a clue.  The buzzing is quite likely to be due to something
like the CPU diving in and out of the interrupt routine, and if that's
true, then it suggests that the counter, decoder, and NAND gate are OK
(otherwise there'd be no interrupt) and the fault is with the MUX or its
connections.

> > The most likely fault is a cracked track near the counter or MUX; I've
> > seen
> > similar situations where a crack right beside an IC becomes heat
> > sensitive
> > (when it warms up, the metal expands and -- counterintuitively --
> > widens
> > the crack.

I should have been a bit clearer there.  I meant the sort of situation
where an IC has a broken leg, or the pad it's soldered to is broken; when
the IC warms up, the other legs expand and lift the IC by a tiny amount,
breaking the connection.

> > If all the keys in one column are faulty, suspect the decoder (7445).
> >
> > If
> > all the keys in several columns are bad, especially suspect the counter
> > (74LS161). If some keys in a column are good but some are bad, but all
> > the
> > keys in a row are bad, suspect the MUX (74LS251).

For benefit of anyone reading the explanation/diagnosis but not the
original symptoms, I ought to point out that the most common keyboard fault
by far is a broken track or two -- but that usually affects part of one
column or part of one row, and virtually never all the keys in several
columns.

-- 
Pete                                           Peter Turnbull
                                               Network Manager
                                               University of York